The present invention relates to an image processing apparatus, and more particularly to a computer graphic apparatus which synthesizes a plurality of images on a virtual screen.
The virtual screen, derived from a memory of a computer, is taken to have an area larger than that of the real screen. When image data on the virtual screen are moved in a vertical or horizontal direction, image data on the real screen is scrolled. In this scroll mode, one end of the virtual screen is connected to the other end if the image on the real screen is scrolled out from the virtual screen, because the virtual screen is limited in size.
FIG. 1 shows a relation between the virtual and real screens in a horizontal scroll mode. When the virtual screen is moved to the right on the figure, a wave-shaped picture illustrated on the virtual screen is moved to the right on the real screen. When one end point "B" of the wave is reached at the real screen, the other point "A" is displayed following the point "B" on the real screen. This process is called an "endless scroll".
FIG. 2 shows the principal of operation of the endless scroll technique. In the endless scroll mode, it may be considered that an image is drawn on the virtual screen shaped in the form of a cylinder, and the image is seen on the real screen by rotating the cylinder continuously. Actually, the virtual screen is also butted together in the vertical direction, so that the virtual screen is formed on a spherical surface rather than a cylindrical surface. This endless scroll mode is called the "Chazutsu" mode.
In a game computer treating many animation images, a background image (BG) and a sprite image (SP) are superimposed on the virtual screen. The background and sprite images are composed of character patterns and sprite patterns, respectively. A position of each character is defined by a raster and a character pitch on the real screen (CRT). Therefore, the background image may be defined by positions, colors and patterns of the characters. The positions of the characters to be displayed are indicated by coordinates on the CRT.
The background image is managed by using a background attribute table (BAT) and character generator (CG) in the memory (RAM), as shown in FIG. 3. The BAT specifies positions and colors of the characters to be displayed. The CG is taken in the RAM, and the CG stores actual character patterns corresponding to CG codes in the BAT.
According to the endless scroll mode, the virtual screen needs an area at least that of the real screen. Further, a plurality of background pictures can not be displayed independently of each other on one screen. If an outside area of the virtual screen is displayed using no endless scroll technique, a disturbed image like a ghost is displayed at the edge portion of the virtual screen.
In the conventional computer, if a plurality of background images BG1 to BG3 are synthesized with each other, each background image needs its own bus (BUS1 to BUS3) and a video encoder to display them, as shown in FIG. 4. In this case, when some background images are selected from BG1 to BG3, each bus needs to be connected to a fader. The background image to be displayed has a 100% brightness, and the others have 0% brightness. According to the system, if the background images are increased in number, a circuitry in the computer becomes complicated, and as a result, the computer must perform much processing.